Check the brake pedal linkage for being too tight, the pedal linkage where it attaches to the master on the inside of the car should be loose without pressure on the pedal, if its compressed at all the drain back ports will be blocked and will not let fluid drain from the calipers back to the reservoir.
You can also try loosening one of the bleeder screws on one of the calipers open, if fluid squirts out like its under pressure and everything releases, theres something stopping fluid from draining back.
Suprabad, you sure theres a check valve in the caliper? pretty sure theres no valving in the calipers besides the bleeders.
Question for original poster, when did this happen? what was changed before it happened? was it something that just happened one day or were the brakes dragging gradually harder untill locking up like they are now?
Another thing, the pressure wont totally release on the rotor, it will be slightly harder to turn the rotor by hand after applying the brakes because the pads will be dragging on the rotor, if you can turn it by hand after pressing the brakes you should be alright. shouldnt take all your strength though.